Method of protection of magnesium and magnesium-base alloys



Patented June 27, 1950 UNITED snares PATENT "OFFICE METHOD .QFPRDTECTION 0F MAGNESIUM AND'MAGNESIUM-BASE :ALLOY S Herbert ManfredFreudiditflean iirasch,

Nanterre, France 'No Drawing. Application Eluly 23, 1941, Serial "No.763,159. Tn'FranceSeptember 22, 1938 :91Clalms; (o1. zoo-5m The presentinvention concerns the treatment of objects made of magnesium metal-oralloys composedprincipa-lly (e. .g. "-85% or-more) of magnesium, forforming said objects a or coating which can protect said articles fromcorrosion. As is we'l1 known, magnesium in the pure state and to asomewhat less extent alloys which contain magnesium associated ialloyed)with small amounts (up to (1 0% or 1'5'%, more or less) of other commonmetals, e. g. Mn, Zn, A1, rare earth metals, etc, are "prone to become'oxidizedor corroded by continued exposure to damp air, air containingtraces of acid vapors, air-containing moisture and salts, or sea water,river water, lake water, etc, and efforts havedongbeen made to protectsuch magnesium or "magnesium alloys, from such oxidation and corrosion.The present invention covers such protection.

Hereinafter, the term magnesimn-base metal will be used to cover suchmetal and alloys-as discussed above.

The objects may be cast, forged, machined, rolled, etc. and maybe of anyappropriate sizes and may be completely made, or not, as desired.

Thearticles (unlessalready wholly clean) should 1 first be degreased,cleaned, washed and dried, which steps may be carried out according toproc- 'esses already known.

The present invention is in' part a continuation of my earlierapplication Ser. No. 297,191, filed Sept. 29, 1939, (which has becomeabandoned).

The present process involves an alternating current electrolytictreatment, in a bathof proper acidity, consisting =principal'lycf anaqueous solution of manganese dichmmate, :tree zfrom substances capable.of attacking the magnesium in such a way as to "bring any magnesium.eom-

pounds into the coating being formed ontheobjects treated, and/or tobring any .substantial' amount of magnesium compounds .intosolution inthe electrolytic bath. .Coatingsof the type set forth herein, whichcontain magnesium com pounds (oxide or salts) are usually porous andonly insecurely attached to .the object treated,

are generally weak and .offer onlyslight protection. This is because MgOis to a slight extent capable of being dissolved by water, especiallywater containing usual amounts of carbon dioxide in solution. Mostso-called" insoluble magnesium salts are *likewise subject to attack bywater especia'lly 'water containing usual amounts ofcarbon dioxideinrsolution.

It is important, in sorder to :avoid chemical .atrtack "of themagnesium, with consequent intro- .duction of such magnesium:compoimdszinto 5 eleotrolyteand/or into the coating being formed on theobject-to avoid the presence in the electrolytezof any substantialamounts of acids which would chemically attack magnesium, to dissolveithe'sa-me, such acids as sulphuric, "phosphoric, nitric, hydrochloric,and many of the common organic acids, anysubstantial amounts, thus beingexcluded *from the electrolyte, as :well :as the salts of such racids.But more slight traces of such acids, even of the strong mineral acids,do 2110 appneciableharm, and hence can .be tolerafted. Hence it :is notnecessary, in "making up the electrolyteto-use thesubstances(iCrOaMnCGa, M1102, etc.) .in'amabsolutely chemicallypurestate, :but theamounts of impurities therein, especially sulphates, .:halides,.-etc.,should not begreat. The definite addition of substantial amounts ofsulphates, phosphates, nitrates, halides, etc, of courseis avoided.

.It is of :course understood that while I preferably use "two articlesor objects of magnesium- :base metal as the two electrodes, and thesetwo articles .are preferably alike and have equal surface areas, it ispermissible to usesuch a magnesium object as one electrode with aninsoluble electrode of electrically conductive material such as carbon,with the production of equally good results. i

In carrying .outthe process, the electrolyte is 'a solution of manganesedichromate, MnCrzOq. This solution should have a pH between 2 and '5,and preferably between 3 and 4. A pH of close to 3:5 is preferable andproduces the best results.

solutionis preferably made by dissolving 10 to 100 grams of chromeanhydride, *CrOz in a liter of water, and adding manganese carbonatethereto,- to give the desired pH. The CO2 of the carbonate is liberatedinto the atmosphere. Hence the carbonic acid does not act to injurethemagnesiu-m. 'Manganese' dioxide, preferably in a freshly precipitatedstate can be similarly dissolved in the chromic acid solution,preferably heated. A part of the oxygen content of-the 'dioxide isliberated during the reaction. Manganese oxalate can be used in asimilar manner, this also liberating CO2 (chromic acid being a strongoxidizer). In this latter case, solution is preferably eiiected, =hot.Mixtures of the manganese compounds can also he used. Manganesecarbonate is the preferred compound to use.

Manganese carbonate, MnCCDa is readily obtainable, is relatively :cheapsand is easily and promptly \dissolved rbyachromic acid solutions andthe action of the .chromic acid "thereupon can substantially 10 about 15minutes.

.10 range 5 to 7 parts (preferably 5.5 tov 6.5,) of.man-

ganese carbonate to 10 parts Ci'Os. This-same proportion (2.4 to 3.4parts of the element manganese to 10 parts CIOs, and preferably 2.6't0f3.1 parts of the element manganese to 10' parts CI'Os) will be observedusing the other manganese. compounds named above as the source of.,man-

ganese ions.

Normal manganese chromate MnC'rOris not readily soluble in water.Manganese dichromate MnCIzOv is sufficiently soluble. It will be'understood that the ratio of manganese compound to C10; as set forth hereinis about the proper proportion for the formation of the MnCrzO-z, withperhaps'a slight excess of CrO3. The electrolyte may contain between2.66% and 12% (e. g. 4% to l0%)' of manganese bichromate (based'on thewhole electrolyte). Without limiting the invention' to the details, I-give thefollowing' examplesr Two electrodes (each a machined plate-likeobject of magnesium metal), of approximately equal size and equalsurface area are immersed in a bath made from 7.5'parts CIO3 and 4.7parts MIlCOa in .100 parts of solution. This gives a solution ofmanganese bichromate, MnCrzOv (perhaps combined with some water). Thetwo electrodes were connected to the A. C. circuit, at 15 volts, and analternating current of 4 amperes per square decimeter was allowed toflow through the cell for The electrolyte was at room temperature (aboutl8-20 0.), throughout said treatment. At the end of this period, theelectrodes were, removed from the solution, wellwashed in running water,dried and examined. It was found that the objects had a stronglyadhering deposit covering their surfaces, which was dark brown, nearlyblack in color, composed of CrQz and MmO4. This coating offered goodpro- .-It .is also possible to somewhat vary the proportion of MnCOs andC1O3 used in making the electrolyte, to give pH values between 2 and 5.

The coatings are less good as the pH of the electrolyte is above orbelow the optimum of 3.5. With lower pH, the coatings are somewhat moreporous. This can be somewhat offsetby agood painting. The coatings canalso be somewhat improved by a subsequent boiling in sodium silicatesolution or by treatment in hot molten parafiin, as shown in mycopending patent applications.

The strength-of the alternating current can vary between 1.5 and 12amperes at a voltage between'4 and 40 volts. The duration of the .A. C.electrolytic treatment can vary substantially. A treatment for 15 tominutes is usually sufiicient to give a sufficiently thick coating.-

I also give the following specific examples which have shown favorableresults.

(a) gr. CrOa, 65 gr. 1VII1CO3, 1 liter of Water. This gives asubstantially 12% solution of manganese bichromate. A. C. treatment at15 volts, 4 amperes per square decimeter of surface of magnesiumor,magnesium wbase alloy object treated, continued 15 minutes at 15-20 C.

(b) 20 gr. CrOa, 12 gr. MnCOa, 1 liter of water. This gives asubstantially 2.66% solution of, manganesebichromate. A. C. at 18 volts,3 amperes, per s q. decimeter of Mg. surface, continued 15 20 minutes at15 to 20 C.

Iclairn 1. A process of protecting magnesium-base metalobjects in anelectrolyte which consists essentially of; an. aqueous solution which isthe liquid reaction product made by dissolving at least one substanceselected from the group consisting of manganese carbonate, manganesedioxide and manganese oxalate, which substance contains from. about 2.4to about 3.4 parts by weight of the element manganese, in an aqueoussolution of chromic acid alone, containing the equivalent of10 parts byweight of chromic anhydride in about 100 to 1,000 partsby weight ofwater, and

applying saidliquid reaction product, while the latter is free from morethan traces of acids which are capable'of chemically attacking metallicmagnesium and dissolving same, and free from more than traces of saltsof'such acids, and while such solution has a pH value between about 2and about ;5, as an electrolytic bath, in which is placed a pair ofelectrodes, at least one of which is such a magnesium-base -metal objectand the other electrode being. formed of an electrically conductivematerial which is insoluble in said electrolyte, and. applying to suchelectrodes an alternating electric current of between 1.5 and 12 amperesper square decimeter of surface of said magnesium-base metal objectbeing treated, at a voltagebetween 4 and 40 volts, at about atmospherictemperature, and continuing the treatment, until a good protective filmsubstantially ,free of magnesium compounds has been formed on saidmagnesium object.

2. ,A process of protecting magnesium-base metal objects which comprisesdissolving a substance. selected from the. group consisting of manganesecarbonate, manganese dioxide and manganese oxalate, which substancecontains about 2.6 toabout 3.1 parts of the element manganese, in anaqueous solution of chromic acid -alone,.containing the equivalent of 10parts of chromic anhydride, and applying said solution, while the latteris free from more than traces of acids which are capable of chemicallyattacking metallic magnesium and dissolving same, and free from morethan traces of salts of such acids, andwhile such solution has a pHvalue of about '3.5,' as an electrolytic bath in which is placed a pairelectrodes, at least one of which is such a magnesium-base metal objectand the other electrodebeing'formed of an electrically conductivematerialwhich is insolube in said electrolyte, and applying to suchelectrodes an alternating electric current of about 4 amperes' persquare decimeter oi surface of said magnesium-base metal object beingtreated, at a voltage of between 10 and 20 volts, at about roomtemperature, and continuing the treatment until a good protectivefilmhas been formed on said object.

' 3. A'proce ss as claimed in claim 1 in which a pair of saidmagnesium-base objects, having about equal surface areas, constitute thetwo electrodes in said electrolytic bath.

4. A process which comprises applying an alternating electric current toan electrolytic cell containing a pair of electrodes in an electrolyte,at least one of said electrodes being an object composed ofmagnesium-base metal, and the other electrode being an electricallyconductive material which is insoluble in said electrolyte, and in whichsaid electrolyte consists essentially of an aqueous solution ofmanganese dichromate having a pH value between about 2 and about 5.

which solution is free from more than traces of acids which would becapable of chemically at tacking metallic magnesium and dissolving same,and free from more than traces of salts of such acids, the saidalternatin electric current being between about 1.5 and 12 amperes persquare decimeter of surface of said magnesium-base metal object beingtreated, and at a voltage between about 4 and about 40 volts, and-continuing said treatment until a good protective film has been formedon said object.

5. A process as in claim 4, in which the pH of the electrolyte is about3.5.

6. For protecting magnesium base articles against corrosion, the step ofsubjectingsuch an article, as an electrode, to alternating currentelectrolysis in a bath consisting essentially of an aqueous solution ofmanganese bichromate, and which bath has a pH between 3 and 4, whichbath is substantially free of acids which can chemically attack metallicmagnesium and substantially free of the salts of such acids.

7. For protecting magnesium base. articles against corrosion, the stepof subjecting such an.

article, as an electrode, to alternating current electrolysis in a bathconsisting essentially of an aqueous solution of manganese bichromate ofabout 4% to about 10% concentration, and which 8. For protectingmagnesium base articles against corrosion, the step of subjecting suchan article, as an electrode, to alternating current electrolysis in abath consisting essentially of an aqueous solution of manganesebichromate of about 2.66% to'about 12% concentration, and which bath hasa pH between 3 and 4, which bath is substantially free of acids whichcan chemically attack magnesium and substantially free of the salts ofsuch acids.

9. In protecting magnesiumbase metal articles, by A. C. electrolysis inan acid chromate solution, the steps of forming the electrolyte by heating maganese oxalate in-a liquid consisting essentially of water andchromic acid until carbon dioxide is liberated, the ratio of chromicacid to manganese oxalate being such as to leave an aqueous solutioncontaining manganese bichromate, and applying alternating electriccurrent to electrodes, at least one of which is magnesium-base metal,while immersed in said aqueous solution as electrolyte.

HERBERT MANFRED FREUD on: JEAN FRASCH.

REFERENCES CITED The following references file of this patent:

UNITED STATES PATENTS are of record in the OTHER REFERENCES Metaux &Corrosion, vol. 14, No. 166 (June 1939), pages 83 through 87.

1. A PROCESS OF PROTECTING MAGNESIUM-BASE METAL OBJECTS IN ANELECTROLYTE WHICH CONSISTS EXSENTIALLY OF AN AQUEOUS SOLUTION WHICH ISTHE LIQUID REACTION PRODUCT MADE BY DISSOLVING AT LEAST ONE SUBSTANCESELECTED FROM THE GROUP CONSISTING OF MANGAESE CARBONATE, MANGANESEDIOXIDE AND MANGANESE OXALATE, WHICH SUBSTANCE CONTAINS FROM ABOUT 2.4TO ABOUT 3.4 PARTS BY WIEGHT OF THE ELEMENT MANGANESE, IN AN AQUEOUSSOLUTION OF CHROMIC ACID ALONE, CONTAINING THE EQUIVALENT OF 10 PARTS BYWEIGHT OF CHROMIC ANHYDRIDE IN ABOUT 100 TO 1,000 PARTS BY WEIGHT OFWATER, AND APPLYING SAID LIQUID REACTION PRODUCT, WHILE THE LATTER ISFREE FROM MORE THAN TRACES OF ACIDS WHICH ARE CAPABLE OF CHEMICALLYATTACKING METALLIC MAGNESIUM AND DISSOLVING SAME, AND FREE FROM MORETHAN TRACES OF SALTS OF SUCH ACIDS, AND WHILE SUCH SOLUTION HAS A PHVALUE BETWEEN ABOUT 2 AND ABOUT 5, AS AN ELECTROLYTIC BATH, IN WHICH ISPLACED A PAIR OF ELECTRODES, AT LEAST ONE OF WHICH IS SUCH AMAGNESIUM-BASE METAL OBJECT AND THE OTHER ELECTRODE BEING FORMED OF ANELECTRICALLY CONDUCTIVE MATERIAL WHICH IS INSOLUBLE IN SLAIDELECTROLYTE, AND APPLYING TO SUCH ELECTRODES AN ALTERNATING ELECTRICCURRENT OF BETWEEN 1.5 AND 12 AMPERES PER SQUARE DECIMETER OF SURFACE OFSAID MAGNESIUM-BASE METAL OBJECT BEING TREATED, AT A VOLTAGE BETWEEN 4AND 40 VOLTS, AT ABOUT ATMOSPHERIC TEMPERATURE, AND CONTINUING THETREATMENT UNTIL A GOOD PROTECTIVE FILM SUBSTANTIALLY FREE OF MAGNESIUMCOMPOUNDS HAS BEEN FORMED ON SLAID MAGNESIUM OBJECT.